This invention relates generally to soft-switching DC/DC converters and specifically to apparatus for clamping the voltage across rectifier diodes in a soft-switching DC/DC converter.
FIG. 1 illustrates a prior art phase shifted bridge DC/DC converter 10 comprised of semiconductor switching devices (xe2x80x9cswitchesxe2x80x9d) S1, S2, S3, and S4, transformer T1 with primary winding NP and secondary windings NS1 and NS2, resonant inductor LL, rectifier diodes D3 and D4, inductor Lo, and an output capacitor C1. Zero voltage turn on for switches S1 and S2 is achieved by using the energy stored in Lo to discharge the capacitance""s of switches S1 and S2 prior to their being turned on. This is the modulated leg of the phase shift bridge. The drive signal to these switches follows the pulse width modulated (PWM) signal of the control circuit.
One known method of achieving zero voltage turn on for switches S3 and S4 is by using the energy stored in resonant inductor LL to discharge the capacitance""s of switches S3 and S4 prior to their being turned on. This is the fixed leg of the phase shift bridge. A secondary function of inductor LL is to control the rate of change of the current (xe2x80x9cdi/dtxe2x80x9d) in the rectifier diodes, reducing their recovery current. By reducing the di/dt in these diodes, electromagnetic interference (xe2x80x9cEMIxe2x80x9d) emissions are also reduced. Inductor LL always needs to be larger than the leakage inductance of transformer T1. The value of inductor LL is preferably determined by the desired power level of the product or the particular circuit application. For instance, for a three (3) kilowatt power converter, resonant inductor LL may need to be between five (5) to ten (10) microH. For a power level of five hundred (500) watts, inductor LL should be in the range of twenty (20) or thirty (30) microH.
The problem with using a linear inductor LL in series with the transformer primary is that, when switches S3 or S4 turn off, the current in resonant inductor LL is interrupted. This causes the voltage at node A in FIG. 1 to have voltage overshoots (xe2x80x9cspikesxe2x80x9d) above VBUS or below ground, depending on the polarity of the current in inductor LL when switch S3 or S4 turns off. This is illustrated in the voltage waveform shown in FIG. 2. The energy stored in the leakage inductance of the transformer increases the magnitude of these voltage spikes. The voltage spikes are reflected to the secondary of transformer T1 and result in voltage spikes across rectifier diodes D3 an D4 when they are not conducting. See the voltage waveform shown in FIG. 3. The magnitude of the voltage spikes across diodes D3 and D4 could exceed the diode breakdown voltage for these diodes, causing diodes D3 and D4 to fail. Prior art methods for eliminating voltage spikes on D3 and D4 have included saturable reactors, RC snubbers, or complicated active clamps.
One current method used to clamp the voltage across diodes D3 and D4 is to connect a first clamping diode between node A and VBUS and a second clamping diode between node A and ground, as shown in phantom in FIG. 1 at D1 and D2. The problem with this topology is that it creates a large forward current and large reverse recovery currents in clamping diodes D1 and D2, which results in substantial power dissipation in these diodes. This prior art solution is disclosed in the Red1, et al. patent, U.S. Pat. No. 5,198,969 (hereafter xe2x80x9cRed1, et al.xe2x80x9d). The topology in Red1, et al. is also effective only at lower frequencies. At narrow duty cycles and frequencies greater than 200 kHz, the clamping diodes used by Red1, et al. suffer from unacceptable reverse recovery losses.
In an article in Intelec 93, entitled: Switch Transitions in the Soft Switching Full-Bridge PWM Phase Shift DC/DC Converter: Analysis and Improvements, Red1 identified these problems of his circuit and proposed the solution of adding a resistor between the clamp diodes and the resonant inductor to discharge the resonant inductor more quickly. However, at high power levels and high frequency, this method creates unwanted dissipation of energy across the resistor.
What is needed is a simpler and more efficient mechanism for clamping the voltage across the rectifier diodes and eliminating rectifier diode failure in a soft-switching converter, while simultaneously minimizing power loss in the converter.
The present invention comprises an apparatus for clamping the voltage across rectifier diodes in a soft-switching DC/DC converter. The DC/DC converter includes a positive and negative input voltage terminal intercoupled by a plurality of semiconductor switching devices, a transformer, a resonant inductor coupled in series with a primary winding of the transformer, and rectifier diodes coupled to a secondary winding of the transformer. The apparatus for clamping the voltage across the rectifier diodes comprises first and second clamp diodes connected in series across the positive and negative input voltage terminals and an additional winding added to the transformer that has fewer turns than the primary winding of the transformer. The additional winding is coupled between the primary winding of the transformer and the junction of the first and second clamp diodes. One preferred embodiment of the present invention is for use with a full bridge converter. Another preferred embodiment of the present invention is for use with a two switch forward converter.